Limits for Recombination in a Low Energy Loss Organic Heterojunction

Abstract

Donor-acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (V) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PCBM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10 cm V s), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the V is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

“…The low FF denotes that as the field across the active layer decreases (i.e., the applied bias approaches V OC ), the balance between charge recombination and charge extraction shifts toward recombination losses. [24] As described below, here we explore the possible effects of contact limitations, report new insights on the blend morphology, and determine the contributions of geminate recombination, nongeminate recombination, temperature and charge transport, from the point of view of limitations on the FF of PIPCP:PC 61 recombination, and charge extraction are schematically represented in Figure 1c-e. [6] On the other hand, Menke et al have shown that PIPCP:PC 61 BM blends are characterized by fast triplet formation from the CT state.…”

“…Instead, the field dependence limiting the FF stems from very fast nongeminate recombination that is competing with charge extraction. [24] Slowing down charge recombination in PIPCP:PC 61 BM is paramount to improving efficiency. As the field increases, the balance shifts toward charge collection (Figure 7b).…”

“…Within the view of a balance between charge recombination and charge extraction, in the absence of a field (at V OC conditions) charge recombination dominates ( Figure 7a). [24] There is evidence in the literature that charge delocalization, achieved by fullerene aggregates, significantly reduces triplet recombination losses, shown by transient absorption. In addition, because ΔEA of PIPCP and PC 61 BM is so low, this condition may lead to the formation of shallow exciton traps that require the assistance of thermal energy or excess electronic energy to escape.…”

“…[45] One possibility is that the deepening of the EA of PIPCP creates PIPCP states with an EA that is even deeper than PC 61 BM, thereby creating shallow trap states. [24] Thermal energy at room-temperature may be sufficient for most excitons to escape the shallow traps, but as thermal energy is reduced the energetic landscape increasingly impedes charge generation. [24] Thermal energy at room-temperature may be sufficient for most excitons to escape the shallow traps, but as thermal energy is reduced the energetic landscape increasingly impedes charge generation.…”

“…Deviations from the Langevin model have been used as evidence for an equilibrium between CT states and free charge carriers, where charges form CT states and dissociate many times before they are finally extracted or lost to recombination. [24] This blend exhibits a relatively unique situation in which charge generation is very efficient, but the bimolecular recombination rate is closely described by the Langevin limit. [58,59] Electron and hole mobilities of PIPCP:PC 61 BM blends, extracted from hole-and electron-only diodes using the spacecharge limited current (SCLC) model, are balanced but modest, at 1 × 10 −4 cm 2 V −1 s −1 ( Figure S7, Supporting Information).…”